Magnetic memory devices are prepared by coating, impregnating or otherwise placing a magnetic composition containing finely-divided magnetic particles onto a substrate, such as a flexible or solid substrate, to prepare a magnetic memory device which is capable of storing and retrieving information. Typical magnetic memory devices would include, but not be limited to, rigid-type devices which may comprise a hard disc composed of a nonmagnetic material, such as aluminum or flexible-type devices which typically would include a floppy disc or a tape composed, for example, of a polyester or other plastic material, and typically Mylar, such as in use for audio tapes, computer tapes or floppy discs. Thus, for example, in the preparation of magnetic tapes, a liquid magnetic composition is coated or impregnated onto a plastic, paper or metal tape, with the magnetic composition containing magnetizable iron oxide particles of single- or multiple-domain magnetic particles which do not lose memory in absence of a magnetic field. In one method of preparation, a liquid magnetic composition is cast or coated onto a polyester film-like material as a wet, controlled solvent-type coating of a thin thickness; for example, 1/10th to 3 mils, and the volatile solvent material is driven off, to provide for a magnetic coating on the substrate surface.
A typical liquid magnetic composition, suitable for use in preparing magnetic memory devices for storage and retrieval of information, would include magnetic compositions which contain finely-divided magnetic particles. These particles, while they may be agglomerated in larger size, typically have a particle size ranging from an average particle size of about 1000 up to 10,000 Angstroms. The magnetic particles may comprise a wide variety of magnetic elements of iron, nickel and cobalt or alloys, and may comprise iron oxide, chromic oxide, barium ferrite, iron particles themselves and various mixtures and alloys thereof, together with various additives.
The magnetic composition also usually includes a small amount ranging from about 0.5% to 5% by weight; for example, about 3% to 4% by weight, of the solids content of the magnetic composition, of a dispersing agent or primary surfactant. One dispersing agent or surfactant in widespread use in the preparation of a liquid magnetic composition comprises deoiled lecithin, which is a mixture of natural phospholipids. The liquid magnetic composition also usually contains an amount of a binder-type material, typically a polymeric-type nonmagnetic binder, such as a urethane or other resin, and usually an elastomeric urethane resin capable of being cross-linked, the binder material adapted to bind the separated finely-divided oxide particles into position and to bind the magnetic oxide particles to the substrate. The amount and nature of the binder material may vary and generally comprise from 5% to 40% by weight of the total solids, and more typically 18% to 35% by weight. Various other additives and modifiers may be added to the magnetic composition as desired, which additives or modifiers include, for example, particulate material to reduce electrostatic charge, such as finely-divided carbon-black material usually in an amount of from about 0.1% to 4% by weight of the solids, and more typically 1.5% to 2.5% by weight. Another modifier includes a lubricant-type material ranging generally from as low as 1% to as high as 8% by weight of the total solids; for example, 2% to 4% by weight, and which may comprise silicones and more typically fatty-acid-type materials like fatty-acid soaps and particularly alkyl stearates and oleates, such as isobutyl stearate. Other additives and modifiers may be used as required, depending upon the nature, type and ultimate use of the magnetic composition.
The magnetic compositions are prepared by placing the ingredients together with a suitable solvent or mixture of solvents and diluents, and more generally volatile organic solvents and diluents, in a ball mill or sand mill. A variety of solvents may be employed, but generally an organic volatile solvent is used, to provide a liquid medium for dispersion of the magnetic particles in the solvent and diluent, and which solvent mixture provides for the solubilization of the binder material and the dispersing agent. Typical solvents and diluents would include benzene, toluene, cyclohexanone, methylethyl ketone, methylisobutyl ketone, dimethyl formamide or tetrahydrofuran alone or in various solvent mixtures. The particulates in the magnetic composition are then dispersed through the use of ball milling, sand milling, ultrasonics or other dispersing-type techiques, so as to provide for the generally uniform dispersion of the finely-divided magnetic particles and to break up the agglomeration of the particles and to disperse any other pigments or particles in the magnetic composition. After the dispersion, the liquid magnetic composition so prepared is then coated or impregnated onto a substrate surface, and the organic solvent and diluents are driven off by heat, to provide a dried coating layer containing the magnetic particles.
Thus, a typical formulation for the preparation of a magnetic composition for use on a substrate, to prepare a memory storage device comprises:
______________________________________ Magnetic metal, metal oxide 10-400 grams or alloy Dispersing agent 1-40 grams Binder 5-300 grams Solvent 5-200 ml ______________________________________
This mixture is dispersed by a grinding operation, and the mixture is then applied to a subtrate surface using various coating and impregnating techniques, such as spin or web coating.
While prior-art techniques and compositions are satisfactory, it is desirable to prepare new and improved liquid magnetic compositions and to prepare magnetic memory devices therefrom which have improved magnetic and other properties, particularly magnetic storage devices which have improved response of the magnetic particles in a magnetic field, so as to obtain or approach a rectangular hysteresis loop.